Dear Sir,

I am having some doubts over thermal time (growing degree days or
heat unit) concept.  General equation for thermal time calculation
is ((Tmax+Tmin)/2)-Tbase, Where Tmax, Tmin are maximum and minimum
temperatures, respectively and Tbase is the base temperature below
which there will be no growth of plant.  Most of the process
oriented crop growth models viz., DSSAT series, ORYZA, SPAW, SWAP
models use thermal concept for defining different phonological
stages as well as for quantifying the developmental rates.  I am
herewith listing different combinations of maximum and minimum
temperatures, for which same thermal time occurs.  All are these
temperature values are hypothetical values, but will represent the
thermal regime of tropical environmental scenario.
------------------------------------------------------
Tmax  Tmin     Tmean  Tbase   Thermal time
(0 C)  0 C)    (0 C)   (0 C)
------------------------------------------------------
30      25      27.5    10              17.5
32      23      27.5    10              17.5
28      27      27.5    10              17.5
34      21      27.5    10              17.5
36      19      27.5    10              17.5
38      17      27.5    10              17.5
40      15      27.5    10              17.5
42      13      27.5    10              17.5
-----------------------------------------------------

 From the above data set, it is clear that there is wide variation
of 28 to 42 0 C of maximum temperature and a variation of  13 and
27 0 C of minimum temperature. But thermal time is same for all
the data sets.  My doubt is how this will be interpreted for
phenology and developmental rate?  It is possible to modify the
thermal time concept with the inclusion of solar radiation or any
other way.

Thank you very much for spending your valuable time for me.  Have
a nice and good day.

Thanking you,

With kind regards,


Sincerely yours,

N. SUBASH
Scientist (Agricultural Meteorology)
ICAR Research Complex for Eastern Region
WALMI Complex,
Phulwari Shariff P.O
PATNA 801 505,
BIHAR
Email address : [log in to unmask]